Petroleum (crude oil) is one of the world's major sources of energy. Crude oil naturally occurs in geological formations. Typically, the crude oil is recovered by simply drilling a well in an oil-bearing formation. For some wells, because the oil is under pressure in the reservoir, the oil rises to the surface unaided and recovery simply involves constructing pipelines to carry the crude oil to storage facilities such as tanks. This is known as primary recovery. Over the life of a well, however, the reservoir pressure falls and eventually becomes insufficient to cause the oil to rise to the surface. In this scenario, additional measures have to be adopted to get the oil to the surface. These additional methods are known as secondary recovery methods. Secondary oil recovery includes: pumping, water injection, natural gas reinjection, and gas lift (air injection, carbon dioxide injection or injection of other gases into the production well).
The primary and secondary oil recovery methods noted above usually do not result in all the oil in a formation being recovered. Indeed, it is estimated that about half to two-thirds of the oil of a formation remains in that formation after primary and secondary oil recovery. To leave that much oil—a finite resource—in each formation is undesirable. Consequently, over time, further methods have been developed to increase the proportion of oil recovered from a formation after primary and secondary methods fail to provide adequate oil production. These methods are known as tertiary or enhanced oil recovery methods. Common enhanced oil recovery methods include thermal enhanced oil recovery such as steam injection and in-situ burning, chemical flooding methods such as polymer flooding, surfactant flooding, alkaline flooding, micellar flooding and alkaline-surfactant-polymer flooding. However, in situ-combustion is hard to control, steam injection requires expensive steam generating equipment, and chemical flooding is often uneconomical because of the cost of the chemicals. Microbial enhanced oil recovery (MEOR) can be used as a secondary or tertiary enhanced oil recovery process that offers an alternative EOR method that is expected to be less costly and potentially more effective than other EOR methods.
MEOR involves the use of biological organisms—microbes—growing in-situ in a formation to facilitate either the production of materials to aid oil recovery or implementing a mechanism for oil recovery. MEOR has been in existence for at least 50 years and is believed to enhance oil recovery in one of or a combination of several ways. First, the microbes produce surfactants in the formation. Surfactants are wetting agents that lower the interfacial tension between fluids and/or substances. Thus, surfactants produced by microbes reduce the interfacial tension of oil droplets that would prevent the oil from moving easily through the formation. Second, the microbes can produce gases such as methane, carbon dioxide, nitrogen and hydrogen. The production of these gases can increase the pressure in the formation and reduce oil viscosity, which makes it easier to mobilize the oil (to the surface). Third, the microbes can also produce compounds, such as acids, that dissolve carbonates and make the formation more permeable so that oil will flow easily thorough the formation to the surface. Fourth, other compounds (solvents) produced by the microbes may decrease the viscosity of the oil so that it flows easier through the formation. Fifth, the microbes break down the hydrocarbons in the oil, making the oil less viscous and easier to recover. Sixth, the microbes may be used to plug certain sections of an oil-bearing formation as a method of modifying fluid flow. These are only some of the ways MEOR is believed to enhance oil recovery.
Microbial processes in the reservoir primarily involve anaerobic microbes, in part, because it is typical that oxygen content is low in oil-bearing formations. Nonetheless, aerobic MEOR is also practiced as is described, for example, in U.S. Pat. No. 5,163,510 entitled, “Method of Microbial Enhanced Oil Recovery,” the disclosure of which is incorporated herein in its entirety by reference. While historically there has been some success with MEOR generally, positive results are not consistent and it may be concluded that MEOR's impact overall, to date, is marginal with regards to the improvement in the proportion of oil recovered from formations. Therefore, there exists a need to make MEOR a more successful method of enhancing oil recovery.